Glasses of high resistivity and low dielectric losses



Patented Jan. 1, 1952 iUN lT E-D STATES PATENT OFF 2,580,562 ICE 2,580,662 GLASSES OF HIGH RESISTIVITY LOW DIELECTRIC LOSSES AndibanzinpParis, France, assignor to Compa'gnie 'Generale de Telegraphic Sans Fil, a .corporation of France 'No Drawing. Application March30, 1949, Serial than ohms per sq. cm. per cm.

The same is'the case with the dielectric losses, which are characterised by the value of tan 5, 5 being the loss angle. For certain glasses, at C. and for frequencies "offthe order of a megacycle, the value of tan 6 reaches and even exceeds 100.10- At hi her frequencies, of the order of 5 megacycles, the value of tan '6 varies bu't'little with the 'freq'uency, or slightly increases when the frequency increases. On the other handgat low frequencies, the valueof tan 5 decreases in general very rapidly with the frequency, :owing to the ionic conductivity of glass.

Apart from the above electric properties, various other physical properties have also to be taken into consideration in the manufacture of a glass: homogeneity of the mass of molten glass before shaping, aptitude for being worked, resistance to devitrification, mechanical strength, resistance to chemical agents, and so forth.

It is known that the electric properties of glasses are improved by reducing their alkaline oxide content and by replacing such oxides generally by oxides of heavy elements such as lead or barium. But the greatest drawback of this method is that it renders the glass difiicult to melt and to shape.

It is also known that glasses of the SiOz, PbO, K20 type have good dielectric properties and a satisfactory aptitude for all methods or shaping. But the properties of such glasses are poor as regards their direct current resistivity, and their dielectric losses have a tendency to increase with the frequency for frequencies above one megacycle.

The present invention relates to a ran e of compositions of glasses which have particularly satisfactory dielectric properties, without impairing their other physical properties of which they also have a sufficient degree.

The glasses according to the invention are characterised by the composition range which is defined as follows:

Percent SI'Qz 45 to 50 K20 8 to 12 CaO+SrO+Ba0* 30 to 36 MgO 0 to 3 B203 4 to 7 Various 0. to 1 The relative proportions of CaO, SrO, BaO, being indiflerent. for instance the composition may include 30-36% BaO without C210 and SrO, these three oxides bang here regarded as physically and chemically equiva- No..84,480. In'France April 23, 1948 .A]:.O3, .ZnOrand PbO zwhich :have

with :"a completeaabsence -of the oxides Na'z'O,

a very unfavorable effect on the .desiredproperties, since it is found that the electric resistivity of glasses increases, roughly, with Ithe sum of the contents of SiOalNaz'O and .AlzOs. It is consequently advantageousito reduce this 'sum as much as possible. i=But it is..not possible'to decrease to too great .an extent the silica SiOz content without running the risk of obtaining a glass which is easily devitrifiableor very difiicult to shape owing tortoo suddena variation of its viscosity for a small difference of temperature. The best course is'therefore to eliminate completely the two "oxides N220 and A1203 from the composition of the glass. Althou h to a lesser extent, the

presence of zinc oxide ZnO also tends to act in the same unfavorable manneras the presence of Na-() and A1203, so that it is also neccessary to eliminate it.

Finally, it has been found that the presence of lead oxide PbO has the effect of producing a marked tendency for the dielectric losses to increase when the frequency is increased above one megacycle. For the use of glasses for highfrequency purposes it is therefore also advantageous to eliminate the lead oxide from such glasses.

On the other hand, contrary to the ideas generally expressed regarding the function of alkali oxides, the oxide K20 acts very favorably in the presence of high alkaline-earth contents and medium boric oxide contents. It is even advantageous to increase to the maximum the K20 content of the glass, while nevertheless remaining within the limits permitted by the increase of expansion due to its presence, since such increase would impair the resistance of the glass to the impact of heat.

Moreover, boric oxide, the presence of which tends to enhance this favorable action of K20, must not be present in excess in the compositicn, since such an excess decreases the level of workability of the glass which in that case becomes very brittle and has a great tendency to harden, thereby impairing its resistance to the impact of heat.

As regards magnesia MgO in the composition, it merely acts to adjust the value of the softening temperatures of the glass, or to prevent the devitrification and facilitate the melting, the refining and the viscosity thereof at the temperatures of operation.

The most essential characteristic property of the types of glasses according to the composition of the invention isv that their dielectric losses decrease when the frequency increases, which enables them to be used in particular in most radio applications in which very high frequency currents are involved.

3 By way of a non-limitative example, the following is a glass composition according to the invention:

Percent SiOz 48.3 K20 9 CaO 3 BaO 32 MgO 2 B203 5.1 AS203 0.6

the physical properties of which may be marized by the following figures:

Resistivity at 200 C. 10 ohms per sq. cm. per cm. Temperature for which the volume resistivity of the glass is 100 megohms per sq. cm. per cm.:

Dielectric loss coefficients:

For 1 megacycle For megacycles tan 6: 7.10- Coeflicient of expansion 95.10 Solubility in water (powder method) 2 8 mgm.

What I claim is:

sum-

tan 6=12.10"

1. Glasses which are more particularly in tended for high-frequency tubes and which consist of: potassium oxide (K), the content of which is between 8 and 12% of the total weight, barium oxide, the content of which is between 30 and 36% of the total weight, boric oxide, the content of which is between 4 and 7% of the total weight, silicon oxide, the content of which is between and of the total weight, magnesium oxide, the content of which is lower than 3% of the total weight, and various fluxes about 1%.

2. Glass comprising:

Alkaline earth oxides CaO, BaO, SrO with indifferent relative proportions 30-36 Magnesia MgO 0- 3 Boric oxide B203 4 7 Various fluxes '0- 1 ANDRE DANZIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re. 23,049 Armistead Nov. 23, 1948 2,429,432 Stanworth Oct. 21, 1947 

1. GLASSES WHICH ARE MORE PARTICULARLY INTENDED FOR HIGH-FREQUENCY TUBES AND WHICH CONSIST OF: POTASSIUM OXIDE (K2O), THE CONTENT OF WHICH IS BETWEEN 8 AND 12% OF THE TOTAL WEIGHT, BARIUM OXIDE, THE CONTENT OF WHICH IS BETWEEN 30 AND 36% OF THE TOTAL WEIGHT, BORIC OXIDE, THE CONTENT OF WHICH IS BETWEEN 4 AND 7% OF THE TOTAL WEIGHT, SILICON OXIDE, THE CONTENT OF WHICH IS BETWEEN 45 AND 50% OF THE TOTAL WEIGHT, MAGNESIUM OXIDE, THE CONTENT OF WHICH IS LOWER THAN 3% OF THE TOTAL WEIGHT, AND VARIOUS FLUXES ABOUT 1%. 